Approach and Results—Lmo2 and Sphk1 knockdown (KD) were performed in Tg(fli1:EGFP)y1 zebra fish and in human umbilical vein EC. Rescue of phenotypes or overexpression of these factors were achieved using mRNA encoding Lmo2 or Sphk1. EC proliferation in vivo was assessed by bromodeoxyuridine immunostaining and fluorescence-activated cell sorter analysis of dissociated Tg(fli1:EGFP)y1 embryos. Cell migration was assessed by scratch assay in human umbilical vein EC and mouse aortic rings. Lmo2 interactions with Sphk1 promoter were assessed by ChIP-PCR. Lmo2 or Sphk1 KD reduced number and length of intersegmental vessels. There was no reduction in the numbers of GFP+ ECs after Lmo2 KD. However, reduced numbers of bromodeoxyuridine+GFP+ nuclei were observed along the dysmorphic intersegmental vessels, accumulating instead at the sprouting origin of the intersegmental vessels. This anomaly was likely because of impaired EC migration, which was confirmed in migration assays using Lmo2 KD human umbilical vein ECs and mouse aortic rings. Both in vivo and in vitro, Lmo2 KD reduced Sphk1 gene expression, associated with less Lmo2 binding to the Sphk1 promoter as assessed by ChIP-PCR. Sphk1 mRNA rescued the Lmo2 KD phenotype.

Conclusions—Our data showed that Lmo2 is necessary for Sphk1 gene expression in ECs. Lmo2 KD reduced Lmo2–Sphk1 gene interaction, impaired intersegmental vessels formation, and reduced cell migration. We identified for the first time Sphk1 as downstream effector of Lmo2.